1. Field of the Invention
The present invention relates generally to unique proto-oncogene polypeptides and specifically to a novel polypeptide of the bcl-2 family and its nucleic acid sequence.
2. Description of the Related Art
Advances in recombinant DNA technology have led to the discovery of normal cellular genes (proto-oncogenes and tumor suppressor genes, and apoptosis/cell death-related genes) which control growth, development, and differentiation. Under certain circumstances, regulation of these genes is altered and normal cells assume neoplastic growth behavior. In some cases, the normal cell phenotype can be restored by various manipulations associated with these genes. There are over 40 known proto-oncogenes and suppressor genes to date, which fall into various categories depending on their functional characteristics. These include, 1) growth factors and growth factor receptors, 2) messengers of intracellular signal transduction pathways, for example, between the cytoplasm and the nucleus, and 3) regulatory proteins influencing gene expression and DNA replication.
Qualitative changes in the structure of proto-oncogenes or their products and quantitative changes in their expression have been documented for several cancers. With chronic myelogenous leukemia, for example, the abl oncogene is translocated to chromosome 22 in the vicinity of the bcr gene. A cancer specific fusion protein, qualitatively different from parent cell proteins, is produced and is an ideal cancer marker. Mutant ras genes have been implicated in the earliest stages of human leukemias and colon cancers. The detection of these mutations in defined premalignant states could provide valuable prognostic information for clinicians.
During their life span, cells normally pass from an immature state with proliferative potential, through sequential stages of differentiation, to eventual cell death. This orderly progression is aberrant in cancer, probably due to alterations in oncogenes, tumor suppressor genes, and other genes. The progression from the immature state to differentiation can be reestablished in inducible leukemia cell lines. For example, ML-1 human myeloblastic leukemia cells can be induced to differentiate to monocytes/macrophages with the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA). The differentiated cells lose proliferative capacity and accumulate in the G.sub.0 /G.sub.1 phase of the cell cycle, while remaining viable and capable of carrying out normal monocyte/macrophage functions. In general, immature, proliferative cells convert to a differentiated, viable, non-proliferative phenotype.
In ML-1 cells, the initial induction or "programming" of this conversion can be separated from the subsequent phenotypic changes. When cells are induced with TPA for three hours under specific conditions, they become irreversibly committed to undergo differentiation over the next three days. This temporal separation can be used to identify genes that increase in expression during the early programming of differentiation. Such "early-induction" genes might influence or help bring about the later phenotypic conversion. Aberrant expression of these early-induction genes, such as the proto-oncogene fos, may lead to development of a transformed phenotype.
Research on oncogenes and their products is motivated partly by the belief that a more fundamental understanding of the mechanisms of cancer causation and maintenance will lead to more rational means of diagnosing and treating malignancies. Using family studies of restriction fragment length polymorphisms (RFLPs) genetically linked to proto-oncogenes, it may be possible to identify cancer-prone individuals.
Current cancer tests are nonspecific and of limited clinical application. For example, a biochemical test, widely used for both diagnostic and monitoring of cancer, measures levels of carcinoembryonic antigen (CEA). CEA is an oncofetal antigen detectable in large amounts in embryonal tissue, but in small amounts in normal adult tissues. Serum of patients with certain gastrointestinal cancers contains elevated CEA levels that can be measured by immunological methods. The amount of CEA in serum correlates with the remission or relapse of these tumors, with the levels decreasing abruptly after surgical removal of the tumor. The return of elevated CEA levels signifies a return of malignant cells. CEA, however, is also a normal glycoprotein found at low levels in nearly all adults. Moreover, this protein can be elevated with several nonmalignant conditions and is not elevated in the presence of many cancers. Therefore, it is far from ideal as a cancer marker.
A similar oncofetal tumor marker is alpha-fetoprotein, an embryonic form of albumin. Again, the antigen is detectable in high amounts in embryonal tissue and in low amounts in normal adults. It is elevated in a number of gastrointestinal malignancies including hepatoma. Like CEA, a decrease correlates with the remission of cancer and a re-elevation with relapse. There is insufficient sensitivity and specificity to make this marker useful for screening for malignancy or for monitoring previously diagnosed cancer in any but a few selected cases.
For years, various therapeutic agents have been used to alter the expression of genes or the translation of their messages into protein products. However, a major problem with these agents is that they tend to act indiscriminately such that healthy cells as well as malignant cells are affected. As a consequence existing chemotherapeutic regimes are often associated with severe side effects due to the non-specific activity of these agents.
One possible approach to specific intentional therapy is by targeting cells expressing particular oncogenes, tumor suppressor genes or apoptosis/cell death genes. Therefore, there is a continual need to identify new oncogenes associated with cancer and neoplastic phenotypes and with the suppression of these phenotypes. Once these genes are identified, specific therapeutics may be designed which are directed, for example, against the genes themselves, their RNA transcripts or their protein products which should have minimal detrimental effect on healthy cells.